High blister resistance and good coating strength are two significant requirements for coated paper used in web offset printing. With the advent of multi-color offset printing and high printing speeds desired to increase productivity, operating temperatures used for print drying (heat-set) ovens have increased. This has further increased the blister resistance requirement for web offset printing paper.
Styrene-butadiene, vinyl acetate (co)polymers and acrylics, including vinyl acrylics, are the three most commonly used synthetic latex binders in paper coatings. Styrene-butadiene and vinyl acetate (co)polymers are used widely because of their lower cost. In general the styrene-butadiene binders provide higher coating strength than vinyl acetate (co)polymers, but vinyl acetate (co)polymers give higher blister resistance. There is a significant need for styrene-butadiene binders which provide significant improvement in blister resistance of coated paper without a significant loss in paper coating strength.
It is well known that for styrene-butadiene binders the blister resistance of coated paper increases as the gel content of the binder is lowered. The coating strength on the other hand decreases when the gel content is lowered. As a consequence, in order to improve one of these two properties (e.g., blister resistance), some sacrifice in the other property (e.g, strength) must be made.
Blister formation in coated paper occurs at relatively high temperature, greater than 150.degree. C., and is a consequence of the presence of water in the paper. When the water vapors generated in the paper at high temperature cannot escape through the coating, the stresses generated exceed the cohesive strength of the paper and blisters result. A number of studies have indicated that blister resistance can be improved by providing a binder that has increased melt flow. It is, however, not possible to have polymers which have sufficiently high covalently bonded gel to provide good strength and have good melt flow at the same time to provide high blister resistance.
An object of the present invention is to provide paper coating latices, particularly styrene-butadiene latices, containing small amounts of functional monomer pairs capable of forming thermally labile bonds, thereby enabling a variation in the apparent gel of the latex binder as a function of temperature. Yet another object of the present invention is to provide paper coating latices containing in polymerized form small amounts of copolymerized acid functional and amine functional monomers. Still another object of the present invention is to provide paper coating latices that contain both the acid monomer and the amine monomer units in each polymer molecule. Another object of the present invention is to provide paper coating latices, particularly styrene butadiene latices, having low gel and therefore good melt flow at high temperatures for high blister resistance and having high gel and, therefore good coating strength at lower temperatures.